Rational Exergy Management Model for Effective Utilization of Low-Enthalpy Geothermal Energy Resources

Abstract

Within the broad range of sustainability and decarbonization efforts, energy and exergy-rational cities are becoming universally important. Within this context, both ORC systems, which are touted as primarily useful for utilizing low-enthalpy geothermal resources and heat pumps, which are considered as the primary tool for decarbonization are critically analyzed in this study. In this context, two cases regarding an ORC, which is used only for power generation without utilizing its waste heat and a heat pump operating on grid power, were examined and was concluded that they are not exergetically sustainable, if they operate as individual systems. This study instead developed an analysis model, which reveals with case studies and examples that a broad hybridization of combining ORC technology, heat pumps, absorption units, thermal storage, and other renewable energy resources, like solar and wind provides sustainable and exergetically rational design solutions. It is argued and verified that, within practical demand and supply constraints in the built environment, such hybrid systems lead to 4th generation district energy systems and beyond, like nearly-zero energy and exergy cities. In order to arrive such conclusions, new evaluation and rating metrics based on Rational Exergy Management Model were introduced. A novel nearly-zero energy and exergy design about a 20000-inhabitant town having geothermal energy potential at a production well-head temperature of 80o C is presented for a simplified purpose of demonstrating the algorithm of the new model This design incorporates ground-source heat pumps, waste heat utilization, cogeneration units, in addition to ORC system. Such an enrichment of the multiple systems even in a simplistic manner in an exergy economy cycle analytically reduces CO2 emissions by about 66%, when compared to a conventional district energy system utilizing natural gas. Yet analyses have shown that results are sensitive upon design constraints and local conditions and concludes that the only option of achieving a truly sustainable solution in terms of exergy towards net-zero status is optimum bundling of the energy resources and systems on a caseby-case design with the main aim of balancing the supply and demand exergy.

Keywords

• ORC technology,Geothermal energy,Hybrid district energy system,Rational Exergy Management Model,CO2 emissions responsibility,Heat pumps,Cogeneration,Thermal energy storage

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